1. Field of Invention
This invention relates generally to a system associated with a gas supply which is adapted to monitor the supply and to indicate its status as gas is drained therefrom, and more particularly, to a system of this type which is installed in a jet aircraft provided with an oxygen supply that is drained only when the plane flies at high cabin altitudes, the system in a dynamic displaying the time in hours and minutes that remain before the oxygen is exhausted based on the current rate of oxygen consumption, while in a static mode the system refers to a database to predict duration based on existing conditions.
2. Status of Prior Art
In propeller-driven aircraft, the propulsion medium is ambient air that is accelerated to the rear of the plane by the action of the rotating propeller. Hence, propeller-driven planes (other than turboprops) do not function efficiently at high altitudes where the propulsion medium is relatively tin.. But a jet aircraft depends on jet propulsion created by a force developed in reaction to the ejection of a high-velocity jet of gas. In the combustion chamber of a jet propulsion engine, combustion of the fuel mixture generates expanding gases which are discharged through an orifice to form the jet. Hence in a jet plane with a bypass engine, where ambient air is not the propulsion medium, the ambient air impedes the forward motion of the plane unless it is bypassed around the combustion zone (that is, the air is funnelled through the engine so that the total mass flow rate through the engine is increased, and hence more thrust is developed).
When a jet plane flies at high altitudes, 35,000 ft (FL 350) for commercial jets and 41,000 ft. (FL 410), the FAA regulations set forth that at least one pilot must have a mask on and be breathing oxygen. Further, the FAA regulations prescribe quantity of oxygen required for the passengers in the cabin and for the flight crew in the flight deck or cockpit. It is for this reason that all commercial jet aircraft are provided with a pressurized oxygen supply in the form of cylinders or bottles. The magnitude of the supply depends on the size of the plane. For example a B-757 jet plane has a single 115 cubic foot oxygen bottle installed thereon which when full has a gas pressure of 1850 psi. On the other hand a B-747 plane has seven such bottles installed thereon. And on a Falcon 50 jet plane there is only one 76.6 cubic foot bottle of oxygen which when the bottle is full has a pressure of 1850 psi. Oxygen canisters (bottles) for aviation are typically cited in pounds per square inch (gauge) of pressure ("psi"), at NTPD (National Temperature Pressure Dry), while oxygen flows for breathing are typically measured volumetrically, usually liters per minute. The oxygen in the canister is supplied to one or more manifolds from where it is distributed to the pilots, crew, and passengers.
The present practice in a jet plane is to provide the oxygen supply installed therein with a pressure gauge coupled to an indicator which informs the pilot in the cockpit of the prevailing pressure of the supply; some systems also compensate for the temperature of the oxygen canister. If when the flight starts and there is 1850 psi in the canister, and sometime later there the reading is 1300 psi, the pilot knows there is less oxygen but does not have a clear indication of the duration of oxygen remaining. What the pressure gauge does not inform the pilot, yet is important that he know, is exactly how much time remains before the oxygen supply is insufficient to meet the oxygen demand of the particular flight. This demand depends not only on the size of the plane and its passenger capacity, but on the actual number of passengers and crew in the flight, and on the type of oxygen masks being used. For example, those used in emergencies when a cabin depressurizes (the yellow-colored drop down masks) are activated by pulling on the mask and the flow is through a fixed valve; thus, the flow through the mask is a function solely of the differential between the oxygen pressure in the manifold and ambient pressure (so more flows as the altitude increases). One the other hand, pilots' masks have demand regulators, so that often the pilots must use reverse breathing (that is, the oxygen is forced under pressure through the mask into the pilot's lungs, and he must force out his exhalation). Accordingly, a pressure gauge reading does not inform the pilot of a jet plane as to the duration of the oxygen supply. It is therefore the present practice to furnish a jet plane pilot with a printed chart or table which he can on occasion consult to determine for a given number of passengers and crew on a particular flight and for a given full supply of oxygen, how much time remains before this supply of oxygen runs out. In fact, each jet plane will have a different oxygen supply system, with different numbers or configurations of manifolds, and different types of regulator masks for pilots.
Pilots, for each flight, are required to plan for sufficient oxygen on board for a worse case scenario. For example, for a flight from New York City to London, most of the trip is over the Atlantic ocean, and the "worst case" is a depressurization at the Equal Time Point (ETP), the point at which the Estimated Time Enroute (ETE) retuning to the nearest diversion airport or continuing to the nearest diversion airport is the same. Based on actual wind and weather conditions, the plane has an effective Ground Speed Return (GSR, returning to the last diversion airport passed) and an effective Ground Speed Continue (GSC, continuing to the nearest diversion airport). ETP can be calculated as EQU ETP=(D.times.GSR).div.(GSC+GSR)
where D is the distance between the GSR diversion airport and the GSC diversion airport (typically measured in nautical miles). The ETP can usually be derived from a computerized flight plan.
Fortunately, the problem of sudden cabin depressurization, to extent of oxygen requirements, is less problematic in a commercial airliner. Oxygen is only required, by FAA regulations, for flight levels above 10,000 ft. (FL 100). Commercial airliners typically carry sufficient fuel so that, after a catastrophic depressurization, they can make an emergency descent to 10,000 ft. and continue or return at that level, avoiding the need for oxygen during the entire ETE to the diversion airport.
The real problem occurs with private (e.g., corporate) jets, where the luxury of uploading sufficient fuel to fly at 10,000 ft. to a diversion airport is lacking. If there is a sudden depressurization and after emergency descent to FL 100 there is sufficient fuel to travel to a diversion airport at FL 100, the problem is averted. Otherwise, the plane must climb to increase the Specific Range (SR). Accordingly, the pilot(s) must calculate, based on the performance charts for the specific aircraft being flown, the minimum altitude at which the SR is sufficient to reach the diversion airport. The higher the altitude, the farther the SR; however, fuel usage increases slightly as altitude increases. Most importantly, at FL greater than 100, oxygen is required for the crew and passengers. Also, wind speed varies as a function of altitude, so a higher altitude may encounter a higher head (or tail) wind speed. Accordingly, in an emergency situation there is a tradeoff among fuel available, altitude (SR), and oxygen available. Unfortunately, determining the altitude for a sufficient SR and the oxygen available is an iterative process, and the time for these calculations is not during a catastrophic depressurization in the middle of the night over water. Additionally, after the decompression, the pilot(s) must determine an operating window to fly to one or two preplanned diversion airports (or, perhaps, an unplanned diversion airport); while the pilot(s) regains control of the aircraft and stabilizes the situation, the jet is still continuing on its flight path, and so is using fuel, oxygen, and changing the distances between it and the diversion airports, further effecting calculations of the operating window.
Ruder, U.S. Pat. No. 3,922,149, is directed to eliminating tanks of stored oxygen by providing an oxygen enrichment system that uses a molecular sieve that absorbs oxygen the least, and hence enriches the oxygen content of the sieve effluent stream.
Bishaf, U.S. Pat. No. 3,875,801 which discloses a pressurized gas tank to supply oxygen to a scuba diver, the gas being depleted at a variable rate. In Bishaf, the amount of gas remaining in the tank is displayed "in terms of the amount of time until depletion." In the Bishaf system, a transducer placed within the gas tank produces an electrical signal indicative of the instantaneous gas pressure. The signal is applied to an integrated circuit chip what develops a signal proportional to, the rate of change of the instantaneous pressure.
Schmitt, U.S. Pat. No. 4,485,669, is directed to a device for determining the timely delivery of compressed gas from compressed gas canisters, and especially for the ejection of weapons in a submarine, to assure proper ejection velocity at all depths at which the submarine may be operating.
Erickson, U.S. Pat. No. 4,408,484, discloses a temperature compensated gauge for pressurized gas, especially for natural gas fuel for vehicles or homes.